Download of DarkNight 1.70 for Mac was available from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The software lies within System Tools, more precisely System Optimization. We recommend checking the downloaded files with any free antivirus. The actual developer of this free Mac application is Hobbyist Software Limited.
From the developer:
Dark Night is an application that automatically switches between light and dark wallpapers based on sunrise and sunset, or on your own schedule. You can instantly see the sunset and sunrise times. You can also run your own Apple Script at sunrise and sunset.
You may want to check out more software for Mac, such as TransparentDock, Visage Login or Peek-a-Boo, which might be related to DarkNight.